1. Field of the Invention
The present patent relates to an oscillator circuit for a semiconductor device, and more particularly to, an oscillator circuit which has a stable period regardless of variations of a process, a power voltage and a temperature.
2. Discussion of Related Art
In general, circuits that must use internal clocks as well as external clocks exist in a memory device and an IC chip. Especially, in a flash memory, a micro-controller or a pump circuit use internal clocks without receiving external clocks. An oscillator circuit generates such internal clocks.
A ring oscillator that is a basic oscillator is formed by connecting an odd number of inverters in series, and feeding back an output from the last terminal as an input of the first inverter. Although very simple in structure, the ring oscillator is influenced by variations of a process, a power voltage and a temperature (PVT variations), and thus a period thereof is seriously varied. In order to solve the above problem, there has been used a circuit which determines a period by RC delay effects by connecting a constant current source to an inverter or adding a resistor, a capacitor, a Schmit trigger or a comparator. However, if a period is varied due to external variations or an area resistance value is varied by a process, a period of an oscillator is also varied.